CPC5620/CPC5621

12

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R05

For single-ended receive applications where only one

RX output is used, the snoop circuit gain can be

adjusted back to 0 dB by changing the value of the

snoop series resistors R6, R7, R44 and R45 from

1.8M

 to 715k This change results in negligible

modification to the ringing detect threshold.

3.3 Off-Hook Operation: OH=0

3.3.1 Receive Signal Path

Signals to and from the telephone network appear on

the tip and ring connections of the application circuit.

Receive signals are extracted from transmit signals by

the LITELINK two-wire to four-wire hybrid. Next, the

receive signal is converted to infrared light by the

receive photodiode amplifier and receive path LED.

The intensity of the light is modulated by the receive

signal and coupled across the electrical isolation

barrier by a reflective dome.

On the equipment’s low voltage side of the barrier, the

receive signal is converted by a photodiode into a

photocurrent. The photocurrent, a linear

representation of the receive signal, is amplified and

converted to a differential voltage output on RX+ and

RX-.

Variations in gain are controlled to within ±0.4 dB by

factory gain trim, which sets the output to unity gain.

To accommodate single-supply operation, LITELINK

Most applications should AC couple the RX outputs as

shown in Figure 4.

LITELINK may be used for differential or single-ended

output as shown in Figure 4. Single-ended use will

produce 6 dB less signal output amplitude. Do not

exceed 0 dBm into 600

 (2.2 V

the standard application circuit. See application note

AN-157, Increased LITELINK III Transmit Power for more

information.

Figure 4. Differential and Single-ended Receive Path Connections to LITELINK

3.3.2 Transmit Signal Path

Connect transmit signals from the low-voltage side

equipment to the TX+ and TX- pins of LITELINK. Do

not exceed a signal level of 0 dBm in 600

 (or 2.2

single-ended signals in LITELINK. The signal is

coupled to the transmit photodiode amplifier in a

similar manner to the receive path. See application

note AN-157, Increased LITELINK III Transmit Power for

more information.

The output of the photodiode amplifier is coupled to a

voltage-to-current converter via a transconductance

stage where the transmit signal modulates the

telephone line loop current. As in the receive path,

gain is set to unity at the factory, limiting insertion loss

variation to ±0.4 dB.

Differential and single-ended transmit signals into

LITELINK should not exceed a signal level of 0 dBm

referenced to 600

 (or 2.2 V

levels above 0dBm consult the application note

AN-157, Increased LITELINK III Transmit Power for more

information.

RX+

RX-

RX+

RX+

RX

RX-

0.1

µF

0.1

µF

0.1

µF

LITELINK

Low-Voltage Side CODEC

or Voice Circuit